Field of the Invention
This invention relates to the field of polymers and processes for their manufacture. More particularly, this invention relates to conjugated polymers including poly(phenylenevinylenes such as poly(2-methoxy,5-(2'-ethyl-hexyloxy)-p-phenylene vinylene), MEH-PPV, which are soluble in common organic solvents for fabrication of shaped articles such as fibers, tapes, rods and films and which are conducting polymers after oxidative doping.
Conducting polymers were discovered in the late 1970's which suggested the possibility of combining the important electronic and optical properties of semiconductors and metals with the attractive mechanical properties and processing advantages of polymers. Initial efforts to this end were discouraging since the new conducting polymers exhibited several undesirable properties including among others insolubility, intractability, relatively poor mechanical properties and moreover such polymers were non-melting.
More recently, specific conjugated polymers systems have been rendered more soluble and processable. For example, the poly(3-alkylthiophene) derivatives (P3ATs) of polythiophene are soluble and meltable with alkyl chains of sufficient length, and the P3ATs have been processed into films and fibers. See for example Hotta, S., et al, Macromolecules, 20:212 (1987); Nowak, M. et al. Macromolecules, 22:2917 (1989); Elsenbaumer, R. L., et al, Synth. Met. 15:169 (1986) and Polym. Mat. Sci. Eng. 53 79 (1985).: Hotta, S. et al, Synth. Met. 26:267 (1988). However, due to the moderate molecular weights and/or the molecular structures of these polymers, the mechanical properties, particularly the modulus and tensile strength of fibers and films made from these polymers are insufficient to enable their use in many applications.
Alternative methods of processing conductive polymers have been developed. For example, poly(phenylenevinylene), PPV, and the alkoxy derivatives of PPV are synthesized via the precursor polymer route. See for example, Wessling, R. A. and Zimmerman, R. G., U.S. Pat. No. 3,401,152 (1968), U.S. Pat. No. 3,706,677 (1972); Gagnon, D. R. et al. Am. Chem. Soc. Polym. Prepr. 25:284 (1984); Momii et al. Chem Lett. (1987); Yamada et al. JCS Chem Commun (1987).
According to the precursor polymer route, a saturated precursor polymer is first synthesized. The precursor polymer is soluble and can be processed into the desired final shape. In the final step, the precursor polymer is thermally converted to the conjugated polymer.
Although the precursor polymer route is advantageous, the multi-step synthesis is complex making the resultant materials relatively expensive which thus limits their utility.
Other efforts have been directed to the development of p-polyphenylenevinylene, PPV, polymers which are soluble in the final conjugated form. Poly(dihexyloxy phenylenevinylene), DHO-PPV, is not soluble in common organic solvents at room temperature but is soluble at temperatures above 80.degree. C. The longer side-chain octyloxy derivative, DOO-PPV, was found to be less soluble in most non-polar solvents, probably because of side chain interdigitation and "side chain crystallization".
Thus, until the present time the limited solubility and inferior mechanical properties of conjugated polymers suitable for oxidative doping has severely limited their use.